0e1afa3eba
A bare `return X;` / `raise` in the middle of a block closed the current
LLVM basic block, but lowerBlock / lowerBlockValue only stopped the
statement loop on the `block_terminated` flag — which lowerReturn
deliberately never sets (it would leak past an `if cond { return }` merge
block). So trailing dead statements were emitted into the already-closed
block, tripping the LLVM verifier with "Terminator found in the middle of
a basic block".
Fix: also stop the statement loop when currentBlockHasTerminator() is
true. That is CFG-level termination of the *current* block, which is
naturally false at an if / inline-if merge block, so conditional returns
still fall through to their trailing statements.
This unblocks ERR E5.1: the canonical failable-closure form
`closure((x) -> (s32,!) { raise error.X; return x; })` has a dead
`return x;` after the unconditional raise and tripped the verifier.
Regression: examples/0038-basic-dead-code-after-terminator.sx.
69 lines
3.0 KiB
Markdown
69 lines
3.0 KiB
Markdown
# 0061 — dead statements after `return` / `raise` emit into a closed block
|
|
|
|
> **✅ RESOLVED (2026-06-01).** Root cause: `lowerBlock` / `lowerBlockValue`
|
|
> ([src/ir/lower.zig](../src/ir/lower.zig)) broke their statement loop only on
|
|
> the `block_terminated` flag, which `lowerReturn` deliberately does NOT set (it
|
|
> would leak past an `if cond { return }` merge block — see the comment at
|
|
> `lowerReturn`). So a bare `return X;` / `raise` mid-block closed the current
|
|
> LLVM basic block while lowering kept emitting the trailing statements into it.
|
|
> Fix: after each `lowerStmt`, also stop the loop when
|
|
> `currentBlockHasTerminator()` is true (CFG-level termination of the *current*
|
|
> block — correctly false at an `if`/`inline if` merge block, so conditional
|
|
> returns still fall through). Regression test:
|
|
> [examples/0038-basic-dead-code-after-terminator.sx](../examples/0038-basic-dead-code-after-terminator.sx).
|
|
|
|
## Symptom
|
|
|
|
Any statement following a block-terminating statement (`return`, `raise`) at the
|
|
same block level is lowered into the basic block *after* its terminator, so the
|
|
LLVM verifier aborts:
|
|
|
|
```
|
|
LLVM verification failed: Terminator found in the middle of a basic block!
|
|
label %entry
|
|
```
|
|
|
|
Observed: a well-formed program with trailing dead code crashes the compiler.
|
|
Expected: the dead statements are dropped (unreachable); the program compiles
|
|
and runs.
|
|
|
|
This blocked ERR E5.1: the canonical failable-closure form from the plan,
|
|
`closure((x) -> (s32, !) { raise error.X; return x; })`, has a dead `return x;`
|
|
after the unconditional `raise` and tripped the verifier.
|
|
|
|
## Reproduction
|
|
|
|
Minimal (non-failable — the bug is general, not error-specific):
|
|
|
|
```sx
|
|
#import "modules/std.sx";
|
|
main :: () -> s32 { return 0; print("dead\n"); }
|
|
```
|
|
|
|
Failable facet (the form that blocked E5.1):
|
|
|
|
```sx
|
|
#import "modules/std.sx";
|
|
E :: error { Neg }
|
|
top :: (x: s64) -> (s64, !E) { raise error.Neg; return x; }
|
|
main :: () -> s32 { print("r={}\n", top(5) catch e 0); return 0; }
|
|
```
|
|
|
|
Both abort with "Terminator found in the middle of a basic block". A
|
|
*conditional* terminator (`if c { return 1; } return 2;`) was unaffected — its
|
|
merge block is fresh and has no terminator.
|
|
|
|
## Investigation prompt
|
|
|
|
The bug is in block-statement lowering in [src/ir/lower.zig](../src/ir/lower.zig):
|
|
`lowerBlock` (~line 1455) and `lowerBlockValue` (~line 1496) iterate `blk.stmts`
|
|
and only check the `block_terminated` flag. `lowerReturn` (~line 1767) emits a
|
|
`ret`/`br` terminator but intentionally does NOT set `block_terminated` (setting
|
|
it would leak past `if cond { return }` merge blocks and wrongly skip their
|
|
trailing statements — see the comment there). The fix is to stop the loop when
|
|
the *current* basic block has a terminator after lowering a statement, using the
|
|
existing `currentBlockHasTerminator()` helper (~line 11725), which is naturally
|
|
false at a merge block. Verify with both repros above (now compile + run) and
|
|
confirm `examples/0518-packs-pack-value-dispatch.sx` (inline-if + return +
|
|
trailing statements) still produces all its output.
|